It has been known for some time that a hot electron transistor could potentially be operated at frequencies in excess of those achievable with conventional (diffusive) transistors. See, for instance, T. E. Bell, IEEE Spectrum, February 1986, pp. 36-38, incorporated herein by reference. Various types of hot electron transistors (HET) have been proposed. This application is concerned with one particular class of such transistors, namely, heterojunction bipolar HETs. For a brief review, see L. F. Eastman, ibid, pp. 42-45, also incorporated herein by reference.
The flow of electrons from emitter to collector in a bipolar transistor is controlled by varying the emitter/base barrier potential by means of an applied voltage V.sub.eb, and is also a function of an externally applied voltage V.sub.bc between base and collector. Under normal operating conditions, V.sub.bc reverse biases the base/collector junction. Electrons injected from the emitter into the base of a bipolar HET have energy substantially greater than the thermal energy of the ambient electrons in the base. These "hot" electrons ideally traverse the base without undergoing significant scattering. If the base/collector barrier is caused to be lower than the hot electron energy then some of the hot electrons can cross the barrier, be transmitted through the depletion region of the collector, and enter the sea of conduction electrons in the collector contact region.
As will be readily understood by those skilled in the art, various difficulties have to be overcome before a device of this type can function as a practical HET. Among these is the difficulty of achieving substantially complete hot electron transport through the base, and, desirably, also through the depletion region of the collector.
Two recently filed U.S. patent applications (Ser. No. 871,494, filed June 6, 1986 by J. R. Hayes et al. and Ser. No. 074,127, filed July 17, 1987, Pat. No. 4829343 by A. F. J. Levi), both incorporated herein by reference, disclose means for achieving improved HETs. However, in view of the general desirability of improved characteristics such as larger .beta. and high cut-off frequency, means for achieving further improvements in HET characteristics would be of considerable significance. This application discloses HETs comprising such means.